Video and audio data have traditionally been recorded and delivered as analog signals. However, digital signals are becoming the transmission medium of choice for video, audio, audiovisual, and multimedia information. Digital audio and video signals are currently delivered widely through digital satellites, digital cable, and computer networks such as local area networks and wide area networks, e.g., the Internet. In addition, digital audio and video signals are currently available on digital media such as audio compact discs, digital audio tape (DAT), minidisc, and laserdisc and digital video disc (DVD) video media. As used herein, a digitized signal refers to a digital signal whose substantive content is generally analog in nature, i.e., can be represented by an analog signal. For example, digital video and digital audio signals are digitized signals since video images and audio content can be represented by analog signals.
A significant disadvantage resulting from the current tremendous growth of digitally stored and delivered audio and video is that digital copies which have exactly the same quality as the original digitized signal can easily be made and distributed without authorization notwithstanding illegality of such copying. The substantive content of digitized signals can have significant proprietary value which is susceptible to considerable diminution as a result of unauthorized duplication.
It is therefore desirable to include identifying data in digitized signals having valuable content such that duplication of the digitized signals also duplicates the identifying data and the source of such duplication can be identified. The identifying data should not result in humanly perceptible changes to the substantive content of the digitized signal when the substantive content is presented to a human viewer as audio and/or video presentations. Since substantial value is in the substantive content itself and in its quality, any humanly perceptible degradation of the substantive content substantially diminishes the value of the digitized signal. Such imperceptible identifying data included in a digitized signal is generally known as a watermark.
In general, adding a watermark to a digitized signal requires significant resources, often exceeding the capacity of server computer systems which deliver digitized signals to individual purchasers. In particular, such server computer systems are typically required to deliver many digitized products simultaneously and have insufficient resources to also add watermarks to those digitized signals prior to, or during, delivery. As a result, watermarks are typically added once by a publishing computer system and the watermarked digitized signal is placed on the server computer system for delivery. The primary disadvantage of such a system is that all copies of a watermarked digitized signal have identical watermark data. Accordingly, a source of unauthorized copies of a watermarked digitized signal cannot generally be determined from such a watermark.
What is needed, therefore, is a watermark system in which identifying data can be securely and robustly included in a digitized signal in a particular efficient manner such that a server computer system can add such data for each delivery of the digitized signal.